Where a large truck, such as a tractor-trailer arrangement, is backed into a loading dock for loading or unloading, it is necessary that the truck be, in some way, anchored in position and prevented from pulling away from the dock. It has been found that simply chocking the wheels is inadequate, and highly dangerous to personnel. As a consequence, numerous restraining devices have been devised, designed to achieve positive restraint while minimizing danger to personnel and to the truck itself.
Typical of such devices are those shown in U.S. Pat. No. 4,472,099 of Hahn et al and U.S. Pat. No. 4,695,216 of Erlandsson. In the Hahn et al arrangement, there is a vertically movable carriage member carrying a hook for engaging the ICC bar of a trailer. The carriage is suspended from a pair of springs, and as the trailer is backed into position, the ICC bar engages the carriage and forces it down, thereby tensioning the springs. The hook is electrically moved into hooking position, and is held there by the motor. Such an arrangement has serious disadvantages, e.g., the constant physical contact between the ICC bar and the carriage results in rapid deterioration and eventual dislodgement of the carriage, the two springs must be matched as to tension, otherwise the carriage may become locked and jammed in its track, and finally, the hook will disengage if there is a failure of hydraulic pressure.
The Erlandsson arrangement likewise uses physical contact between the carriage and the ICC bar, wherein the ICC bar rides up a sloping face of the carriage to the hooked position. Such an arrangement is easily broken or damaged by this contact and, as a consequence, is not completely reliable.
In U.S. Pat. No. 4,759,678 of Hageman, there is shown an arrangement that utilizes a movable carriage carrying a hook or restraining member. The carriage is moved vertically by means of a pneumatic actuated piston, which also moves the hook into operative position. The carriage and hook are maintained in operative position by the air pressure on the piston. Hence a leak in the fluid supply, for example, can result in the hook disconnecting prematurely. This unit also involves expensive installation fees.
When a truck is being loaded, the bed of the truck will be lowered as the load is increased. In a similar manner, the bed of the truck will rise as the load is decreased. It is desirable that this action, known as "float", be compensated for to insure that the restraining member does not disengage. In both the Hageman and the Hahn et al arrangements, provision is made for compensating for the float. The springs of Hahn et al compensate to an extent, but, as pointed out, unless the springs are virtually identical, locking and jamming of the carriage can occur. Hageman's pneumatic piston likewise compensates for float due to the compressibility of the air, but, as pointed out, leakage or loss of air can result in failure of the system.
The problems and dangers associated with the use of pneumatic, fluid or hydraulic drives can be eliminated by the use of direct, positive drives. Arrangements utilizing positive drives are shown in U.S. Pat. No. 4,589,813 of Hagen et al and U.S. Pat. No. 4,735,542 of Fisher. The Hagen et al arrangement utilized a rack and pinion to drive a restraining bar into its operative position, while the Fisher arrangement utilizes, in one embodiment, a jack screw upon which is mounted a screw follower which is connected, in turn, either directly or indirectly, to the restraining member. The Hagen et al arrangement exposes the drive means, i.e., the rack, to the ICC bar, by which it can easily be damaged. The Fisher arrangement is configured to be sunk into the ground, which creates problems of access, cleanliness and the like. Neither structure is adapted to accommodate float and compensate therefor.